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Apicomplexa
| subdivision_ranks = Classes & Subclasses Perkins, 2000 | subdivision = * (= Hematozoa) * **Coccidia (= ) ** (= Gregarinasina) }} The Apicomplexa (also called Apicomplexia) are a large 门 (生物) of 寄生 囊泡虫s. Most of them possess a unique form of 細胞器 that comprises a type of 色素體 called an 顶质体, and an apical complex structure. The organelle is an adaptation that the apicomplexan applies in penetration of a host cell. The Apicomplexa are unicellular and spore-forming. All species are endoparasites of animals, except , a 共生 in marine animals, originally classified as a 壶菌门 fungus. Motile structures such as 鞭毛 or 偽足s are present only in certain 配子 stages. The Apicomplexa are a diverse group that includes organisms such as the coccidia, gregarines, 梨形蟲目s, s, and . Diseases caused by Apicomplexa include: * (巴倍虫属) * 疟疾 (瘧原蟲) * 隐孢子虫病 (小隱孢子蟲) * ( ) * ( (formerly known as "Isospora Belli")) * 弓蟲症 (弓形虫) The name of the taxon Apicomplexa derives from two 拉丁语 words—''apex'' (top) and complexus (infolds)—and refers to a set of 細胞器s in the 頂複門的生命週期. The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group of parasitic protozoans, in general without flagella, cilia, or pseudopods. Most of the Apicomplexa are motile, however, by use of a that uses adhesions and small static myosin motors. The other main lines were the (now in 有孔蟲界), the 黏體動物 (now known to be highly derived 刺胞動物門 动物s), and the 微孢子蟲 (now known to be derived from 真菌). Sometimes, the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset. Description Apicomplexa consists of all eukaryotes that contain a group of structures and organelles collectively termed the 頂複門. This complex consists of structural components and 分泌 that are required for invasion of 宿主 细胞s during the parasitic stages of the Apicomplexan life cycle. Apicomplexa have complex life cycles, involving several stages and typically undergoing both 无性生殖 and 有性生殖. All Apicomplexa are s for some portion of their life cycle, with some parasitizing two separate hosts for their asexual and sexual stages. Besides the conserved apical complex, Apicomplexa are morphologically diverse. Different organisms within Apicomplexa, as well as different life stages for a given apicomplexan, can vary substantially in size, shape, and subcellular structure. All members of this phylum have an infectious stage—the sporozoite—which possesses three distinct structures in an apical complex. The apical complex consists of a set of spirally arranged 微管s (the ), a secretory body (the ) and one or more polar rings. Additional slender electron-dense secretory bodies ( s) surrounded by one or two polar rings may also be present. This structure gives the phylum its name. A further group of spherical organelles is distributed throughout the cell rather than being localized at the 頂複門 and are known as the dense granules. These typically have a mean diameter around 0.7 μm. Secretion of the dense-granule content takes place after parasite invasion and localization within the and persists for several minutes. Other morphological findings common to all members of this phylum include: *The 细胞核 is 染色體倍性. *鞭毛 are found only in the motile gamete. These are posteriorly directed and vary in number (usually one to three). *基体 are present. Although hemosporidians and piroplasmids have normal triplets of 微管s in their basal bodies, coccidians and gregarines have nine singlets. *The 線粒體 have tubular 线粒体嵴. *A 高尔基体 is present. *中心粒s, 叶绿体s, ejectile organelles, and inclusions are absent. *Colourless 色素體s are present in some species. *The cell is surrounded by a 原生動物 of three membrane layers (the alveolar structure) penetrated by micropores. Replication: *有絲分裂 is usually closed, with an intranuclear spindle; in some species, it is open at the poles. *Cell division is usually by 分裂 (生物学). *减数分裂 occurs in the 受精卵. Mobility: Apicomplexans have a unique gliding capability which enables them to cross through tissues and enter and leave their host cells. This gliding ability is made possible by the use of adhesions and small static myosin motors. Other features common to this phylum are a lack of cilia, sexual reproduction, use of micropores for feeding, and the production of oocysts containing sporozoites as the infective form. Most apicomplexans have an 顶质体, (a nonphotosynthetic plastid) and mitochondrial and s, although species and are possible exceptions, as they are thought to have lost their plastids after the diverging 最近共同祖先 of apicomplexans. General features of the subgroups Within this phylum are three groups—coccidians, gregarines, and haemosporidians. The coccidians and gregarines appear to be relatively closely related. Perkinsus '', while once considered a member of the Apicomplexa, has been moved to a new phylum — . Gregarines The gregarines are generally parasites of 环节动物门s, 节肢动物s, and mollusks. They are often found in the 消化道s of their hosts, but may invade the other tissues. In the typical gregarine lifecycle, a 活動體 develops within a host cell into a schizont. This then divides into a number of merozoites by 分裂 (生物学). The merozoites are released by lysing the host cell, which in turn invade other cells. At some point in the apicomplexan lifecycle, are formed. These are released by lysis of the host cells, which group together. Each gametocyte forms multiple 配子s. The gametes fuse with another to form oocysts. The oocysts leave the host to be taken up by a new host. Coccidians -->弓形虫 (Coccidia) parasites]] In general, coccidians are parasites of 脊椎动物s. Like gregarines, they are commonly parasites of the 上皮組織 cells of the gut, but may infect other tissues. The coccidian lifecycle involves merogony, gametogony, and sporogony. While similar to that of the gregarines it differs in 受精卵 formation. Some trophozoites enlarge and become , whereas others divide repeatedly to form s (anisogamy). The microgametes are motile and must reach the macrogamete to fertilize it. The fertilized macrogamete forms a zygote that in its turn forms an oocyst that is normally released from the body. Syzygy, when it occurs, involves markedly anisogamous gametes. The lifecycle is typically haploid, with the only diploid stage occurring in the zygote, which is normally short-lived. The main difference between the coccidians and the gregarines is in the gamonts. In the coccidia, these are small, intracellular, and without epimerites or s. In the gregarines, these are large, extracellular, and possess epimerites or mucrons. A second difference between the coccidia and the gregarines also lies in the gamonts. In the coccidians, a single gamont becomes a macrogametocyte, whereas in the gregarines, the gamonts give rise to multiple gametocytes. Haemosporidia -->間日瘧原蟲'' (Haemosporidia) parasite among human red blood cells]] The Haemosporidia have more complex lifecycles that alternate between an arthropod and a vertebrate host. The trophozoite parasitises 红血球s or other tissues in the vertebrate host. Microgametes and macrogametes are always found in the blood. The gametes are taken up by the insect vector during a blood meal. The microgametes migrate within the gut of the insect vector and fuse with the macrogametes. The fertilized macrogamete now becomes an 頂複門的生命週期, which penetrates the body of the vector. The ookinete then transforms into an oocyst and divides initially by meiosis and then by mitosis (haplontic lifecycle) to give rise to the 頂複門的生命週期s. The sporozoites escape from the oocyst and migrate within the body of the vector to the salivary glands where they are injected into the new vertebrate host when the insect vector feeds again. Reproduction and lifecycle Most members have a complex lifecycle, involving both asexual and sexual reproduction. Typically, a host is infected via an active invasion by the parasites (similar to ), which divide to produce 頂複門的生命週期s that enter its cells. Eventually, the cells burst, releasing merozoites, which infect new cells. This may occur several times, until gamonts are produced, forming gametes that fuse to create new cysts. Many variations occur on this basic pattern, however, and many Apicomplexa have more than one host. The apical complex includes 囊泡s called and s, which open at the anterior of the cell. These secrete enzymes that allow the parasite to enter other cells. The tip is surrounded by a band of 微管s, called the polar ring, and among the Conoidasida is also a funnel of tubulin proteins called the conoid. Over the rest of the cell, except for a diminished mouth called the micropore, the membrane is supported by vesicles called alveoli, forming a semirigid pellicle. The presence of alveoli and other traits place the Apicomplexa among a group called the 囊泡虫s. Several related flagellates, such as and , have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of the 双鞭毛虫门s. They are probably similar to the common ancestor of the two groups. Another similarity is that many apicomplexan cells contain a single 色素體, called the 顶质体, surrounded by either three or four membranes. Its functions are thought to include tasks such as lipid and heme biosynthesis, and it appears to be necessary for survival. In general, plastids are considered to have a common origin with the chloroplasts of dinoflagellates, and evidence points to an origin from 紅藻門 rather than 綠藻. Parasitology and genomics -->弓形虫, transmission electron microscopy]] Many of the apicomplexan parasites are important pathogens of human and domestic animals. In contrast to 细菌l pathogens, these apicomplexan parasites are 真核生物 and share many metabolic pathways with their animal hosts. This makes therapeutic target development extremely difficult – a drug that harms an apicomplexan parasite is also likely to harm its human host. At present, no effective 疫苗s are available for most diseases caused by these parasites. Biomedical research on these parasites is challenging because it is often difficult, if not impossible, to maintain live parasite cultures in the laboratory and to genetically manipulate these organisms. In recent years, several of the apicomplexan species have been selected for . The availability of genome sequences provides a new opportunity for scientists to learn more about the 演化 and biochemical capacity of these parasites. The predominant source of this genomic information is the family of websites, which currently provides specialised services for ''瘧原蟲 species ( ), coccidians (ToxoDB), 梨形蟲目 (PiroplasmaDB), and species (CryptoDB). One possible target for drugs is the plastid, and in fact existing drugs such as 四环素类抗生素s, which are effective against apicomplexans, seem to operate against the plastid. Blood-borne genera Within the Apicomplexa are three suborders of parasites: * suborder —eight genera * suborder —all genera in this suborder * suborder Eimeriorina—two genera ( and ) Within the Adelorina are species that infect 无脊椎动物s and others that infect 脊椎动物s. The Eimeriorina—the largest suborder in this phylum—the lifecycle involves both sexual and asexual stages. The asexual stages reproduce by schizogony. The male gametocyte produces a large number of gametes and the zygote gives rise to an oocyst, which is the infective stage. The majority are monoxenous (infect one host only), but a few are heteroxenous (lifecycle involves two or more hosts). The number of families in this later suborder is debated, with the number of families being between one and 20 depending on the authority and the number of genera being between 19 and 25. Evolution All members of this phylum are parasitic and evolved from a free-living ancestor. This lifestyle is presumed to have evolved at the time of the divergence of dinoflagellates and apicomplexans. Further evolution of this phylum has been estimated to have occurred about . The oldest extant clade is thought to be the archigregarines. Many Coccidiomorpha have an 宿主, as well as a primary host, and the evolution of hosts proceeded in different ways and at different times in these groups. For some coccidiomorphs, the original host has become the intermediate host, whereas in others it has become the definitive host. In the genera , , , , and 弓形虫, the original is now definitive, whereas in Akiba, , 巴倍虫属, , , , Karyolysus, , 瘧原蟲, , and , the original hosts are now intermediate. Similar strategies to increase the likelihood of transmission have evolved in multiple genera. Polyenergid oocysts and tissue cysts are found in representatives of the orders and 艾美亞目. 瘧原蟲s are found in and most species of 瘧原蟲; transovarial transmission of parasites occurs in lifecycles of Karyolysus and 巴倍虫属. Horizontal gene transfer appears to have occurred early on in this phylum's evolution with the transfer of a lysine 20 (H4K20) 組織蛋白, (Set8), from an animal host to the ancestor of apicomplexans. A second gene—H3K36 methyltransferase (Ashr3 in 植物s)—may have also be horizontally transferred. Phylogenetic relations This has rarely been studied at the subclass level. The Haemosporidia are related to the gregarines and the piroplasms and coccidians are sister groups. The Haemosporidia and the Piroplasma appear to be sister clades and are more closely related to the coccidians than to the gregarines. Transposons appear to be rare in this phylum, but have been identified in the genera Ascogregarina and . Taxonomy History The first Apicomplexa protozoan was seen by 安東尼·范·列文虎克, who in 1674 saw probably oocysts of in the 胆囊 of a 兔. The first species of the 门 (生物) to be described, in 蠼螋s intestines, was named by Dufour in 1828. He thought that they were a peculiar group related to the 吸蟲綱s, at that time included in . Since then, many more have been identified and named. During 1826–1850, 41 species and six genera of Apicomplexa were named. In 1951–1975, 1873 new species and 83 new genera were added. The older taxon Sporozoa, included in 原生動物, was created by in 1879 and adopted by in 1880.Bütschli, O. (1880-82). Dr. H.G. Bronn's Klassen und Ordnungen des Thier-Reichs. Erster Band: Protozoa. Abt. I, Sarkodina und Sporozoa, https://archive.org/details/drhgbronnsklasse0101bron. Through history, it grouped with the current Apicomplexa many unrelated groups. For example, Kudo (1954) included in the Sporozoa species of the (有孔蟲界), 微孢子蟲 (真菌), 黏體動物 (动物), and (綠藻門), while Zierdt (1978) included the genus (不等鞭毛類). was also thought to be sporozoan. Not all of these groups had spores, but all were parasitic. However, other parasitic or symbiotic unicellular organisms were included too in protozoan groups outside Sporozoa ( , 纤毛虫 and 变形体), if they had flagella (e.g., many 動質體, , , , ), cilia (e.g., ) or pseudopods (e.g., 内阿米巴属, 棘变形虫属, ). If they had cell walls, they also could be included in plant kingdom between 细菌 or 酵母s. Sporozoa is no longer regarded as biologically valid and its use is discouraged, although some authors still use it as a synonym for the Apicomplexa. More recently, other groups were excluded from Apicomplexa, e.g., and (now in Protalveolata). The field of classifying Apicomplexa is in flux and classification has changed throughout the years since it was formally named in 1970. By 1987, a comprehensive survey of the phylum was completed: in all, 4516 species and 339 genera had been named. They consisted of: * Class ** Subclass Gregarinasina 異名 *** Order , with 1624 named species and 231 named genera ** Subclass p.p *** Order 真球蟲目 p.p **** Suborder p.p ***** Group s, with 399 species and four genera **** Suborder Eimeriorina, with 1771 species and 43 genera * Class ** Order , with 444 species and nine genera ** Order 梨形蟲目, with 173 species and 20 genera * Other minor groups omitted above, with 105 species and 32 genera Although considerable revision of this phylum has been done (the order Haemosporidia now has 17 genera rather than 9), these numbers are probably still approximately correct. Jacques Euzéby (1988) in 1988 created a new class by merging subclass Piroplasmasina and suborder . *Subclass Gregarinasina (the gregarines) *Subclass ** Suborder (the adeleorins) ** Suborder Eimeriorina (the eimeriorins) *Subclass ** Order ** Order The division into Achromatorida and Chromatorida, although proposed on morphological grounds, may have a biological basis, as the ability to store 瘧色素 appears to have evolved only once. Roberts and Janovy (1996) Roberts and Janovy in 1996 divided the phylum into the following subclasses and suborders (omitting classes and orders): *Subclass Gregarinasina (the gregarines) *Subclass ** Suborder (the adeleorins) ** Suborder Eimeriorina (the eimeriorins) ** Suborder (the haemospororins) *Subclass Piroplasmasina (the piroplasms) These form the following five taxonomic groups: # The gregarines are, in general, one-host parasites of invertebrates. # The adeleorins are one-host parasites of invertebrates or vertebrates, or two-host parasites that alternately infect haematophagous (blood-feeding) invertebrates and the blood of vertebrates. # The eimeriorins are a diverse group that includes one host species of invertebrates, two-host species of invertebrates, one-host species of vertebrates and two-host species of vertebrates. The eimeriorins are frequently called the coccidia. This term is often used to include the adeleorins. # Haemospororins, often known as the malaria parasites, are two-host Apicomplexa that parasitize blood-feeding 雙翅目 flies and the blood of various tetrapod vertebrates. # Piroplasms where all the species included are two-host parasites infecting ticks and vertebrates. Perkins (2000) This scheme is taken from Perkins et al. It is outdated as the have since been recognised as a sister group to the dinoflagellates rather that the Apicomplexia. The remainder of the scheme appears to be valid: *Class *:Conoid present only in the 頂複門的生命週期 of some species ::*Order :::Macrogamete and microgamete develop separately. Syzygy does not occur. Ookinete has a conoid. Sporozoites have three walls. Heteroxenous: alternates between vertebrate host (in which merogony occurs) and invertebrate host (in which sporogony occurs). Usually blood parasites, transmitted by blood-sucking insects. ::*Order 梨形蟲目 *Class **Subclass Gregarinasina ***Order ***Order ****Suborder ****Suborder Eimeriorina ***Order **Subclass ***Order ***Order 真球蟲目 ***Order ***Order *Class ::*Order Perkinsorida :::*Family The name Protospiromonadida has been proposed for the common ancestor of the Gregarinomorpha and Coccidiomorpha. References External links * * Category:頂複門 Category:SAR supergroup phyla Category:Apicomplexa